To expand on what homeomorphic said I'll give an example of a simple differential equation

[itex]\frac{df}{dx}(x) = 1[/itex]

This means that for every x the rate of change of f with respect to x is 1, this is also the same as saying the 'slope' of the graph of f(x) is always 1. The solution to this equation is f(x) = x.

[itex]\frac{df}{dx}(x) = x[/itex]

This means that the slope of the graph of f(x) is always equal to x, at x=1 the slope of f is 1, at x=10 the slope is 10. The solution to this one is [itex]f(x) = 2x^2[/itex] which is a little harder to see without knowing any calculus.

The differential part just refers to the 'd' part in the equations, df is sometimes called the differential of f.

I understand what an equation is.
I bet an equation with variable(s), is a function (maybe formula too?).

I cant get "differential" part.
Is y=a*x+b a differential one ?

Do you know what a function is? Let's stay in 1D for a moment. A function there is just some expression, formula, or rule that takes an input number (or a variable representing such a number) and spits out another number.

Do you know what the derivative of a function is? It gives an idea of how fast the function's output changes with respect to changes in its input.

Differential equations is the study of equations involving at least one derivative of a function, and usually based on just this information, we're trying to reconstruct what the original function might be or otherwise study the properties of such a function.

To expand on what homeomorphic said I'll give an example of a simple differential equation

[itex]\frac{df}{dx}(x) = 1[/itex]

This means that for every x the rate of change of f with respect to x is 1, this is also the same as saying the 'slope' of the graph of f(x) is always 1. The solution to this equation is f(x) = x.

[itex]f(x) = x + C[/itex]

[itex]\frac{df}{dx}(x) = x[/itex]

This means that the slope of the graph of f(x) is always equal to x, at x=1 the slope of f is 1, at x=10 the slope is 10. The solution to this one is [itex]f(x) = 2x^2[/itex] which is a little harder to see without knowing any calculus.

Eg if a variable changes like 2, 4, 6, 8, ... the rate of its change is 2?

Also, equation with two or more variables inside is a function? (Ι bet there cannot be only one variable inside an equation and all the other parts of the equation being constants)

Derivative is the rate of change of a function.

For example, one might have a function [itex]f(x) = x^2[/itex]. This means the function [itex]f[/itex] takes whatever it is given and spits out the square. Hence, [itex]f(1) = 1[/itex], [itex]f(1.5) = 2.25[/itex], and [itex]f(\pi/2) = \pi^2/4[/itex].

You can have linear functions. Let's say [itex]g(x) = 2x[/itex]. This defines a function so that [itex]g(1) = 2[/itex], [itex]g(2) = 4[/itex], [itex]g(3) = 6[/itex], and so on, but usually, unless one explicitly says so, it's perfectly fine to feed the function g a non-integer argument. In this case, [itex]g(1.5) = 3[/itex], for instance.

To be honest, if you're unfamiliar with functions, I would study those for a while first before trying to understand derivatives and differential equations.

Eg if a variable changes like 2, 4, 6, 8, ... the rate of its change is 2?

Also, equation with two or more variables inside is a function? (Ι bet there cannot be only one variable inside an equation and all the other parts of the equation being constants)

Think about it like driving along the road, and recording how far you are from where you started. So at t = 0, when you first start recording, you're 0 miles from... somewhere. At another time, say t = 10 (call it minutes), you're now 5 miles from that initial spot.

At any instant, you can record some distance you've traveled and plot it. If you're always going away from the start point (so your distance is always increasing), you have your distance -- we'll call it "d" -- as a function of time, t. Or, d = f(t).

At the end, if you divided your total distance by the total time, you'd have your average velocity. Say for example you went 100 miles in 120 minutes (2 hours). That means you averaged 50 mph. In real life you're not going to go the same speed all the time, sometimes maybe 20 mph (hope I'm not stuck behind you), sometimes 75 mph, etc.

So if you took the distance you'd traveled in 2 minutes (instead of the whole time), say between 2 and 4 minutes, you'd have your average velocity over that timespan. If you make that shorter and shorter (so your timespan of interest approaches zero), you are now finding the derivative (the instantaneous rate of change -- which in this case if velocity) of your distance function.

Don’t make things to difficult. It is just a simple equation. There are of course two ways to understand Des (from the math point also from the application point of view (but this is difficult) that’s what previous writer try to explain) .

So, what is the solution of
[itex] 3\cdot b = 6 [/itex] Every kid from play school knows the answer what b is.

It is the same with Des. What is the solution of
[itex] x’(t) = x(t)[/itex] .... ??? [itex] x(t) = ? [/itex]

The problem is that we have people unfamliar with what [itex]x'(t)[/itex] means (it denotes the first derivative of the function [itex]x(t)[/itex], and is also denoted [itex]\frac{dx}{dt}[/itex], among other ways).